Support saving the TODO list content in the session file

[naoliv]

It would be extremely welcome if we could save a session and also have the items in our TODO list included on it (so opening a session would restore the list, allowing us to fix/verify whatever was pending).

JOSM:

Relative:URL: ^/trunk
Repository:UUID: 0c6e7542-c601-0410-84e7-c038aed88b3b
Last:Changed Date: 2018-11-28 01:09:01 +0100 (Wed, 28 Nov 2018)
Revision:14460
Build-Date:2018-11-28 00:26:41
URL:https://josm.openstreetmap.de/svn/trunk

Identification: JOSM/1.5 (14460 en) Linux Debian GNU/Linux testing (buster)
Memory Usage: 855 MB / 6144 MB (245 MB allocated, but free)
Java version: 11.0.1+13-Debian-2, Oracle Corporation, OpenJDK 64-Bit Server VM
Screen: :0.0 1600x900, :0.1 1280x1024
Maximum Screen Size: 1600x1024
Java package: openjdk-11-jre:amd64-11.0.1+13-2
Java ATK Wrapper package: libatk-wrapper-java:all-0.33.3-21
VM arguments: [-Dawt.useSystemAAFontSettings=gasp]
Program arguments: [--language=en]
Dataset consistency test: No problems found

Plugins:
+ OpeningHoursEditor (34535)
+ PicLayer (34544)
+ SimplifyArea (34586)
+ apache-commons (34506)
+ buildings_tools (34724)
+ download_along (34503)
+ ejml (34389)
+ geojson (87)
+ geotools (34513)
+ jaxb (34506)
+ jts (34524)
+ log4j (34527)
+ measurement (34529)
+ merge-overlap (34664)
+ opendata (34698)
+ poly (34546)
+ reverter (34552)
+ tageditor (34560)
+ todo (30306)
+ turnlanes-tagging (272)
+ turnrestrictions (34643)
+ undelete (34568)
+ utilsplugin2 (34506)